1. Field of the Invention
The present invention relates to an antenna, and in particular, to a printed-dipole antenna array and a method for forming a printed-dipole array antenna.
2. Description of the Related Art
In general, a printed-dipole antenna array is utilized in wideband communication systems (e.g., point-to-point, radio relay, cellular, PCS: Personal Communication Service, and satellite communications), radars, and electromagnetic support measurement (ESM) and electromagnetic counter measurement (ECM) systems.
The printed array antenna technology enables a lightweight and low-cost antenna structure to be achieved. One of the most popular elements in printed arrays is the microstrip dipole using a wide frequency range from ultra high frequency (UHF) to K.sub.a band (see R. J. Mailloux, Phased Array Antenna Handbook, Artech House, 1994, p.251). At pages 310 and 311 of the above-mentioned Phased Array Antenna Handbook is described a conventional microstrip dipole array, fully available with low-cost fabrication. Microstrip dipoles and a microstrip corporate feed having phase shifters and other integrated devices are etched together on the same printed circuit board (PCB). In Antenna Engineering Handbook by R. C. Johnson, 3rd edition, McGraw Hill, NY, 1993 (pp. 32-22 and 20-29), two other samples of printed-dipole array antennas with a similar architecture are described.
FIG. 1 is a schematic perspective view of a conventional printed-dipole antenna array (see Phased Array Antenna Handbook, FIG. 5.28A). In FIG. 1, printed circuit boards (PCBs) 10 each having microstrip dipoles 12 and a feed 14 are installed parallel to each other and perpendicular to a common flat ground screen 18 providing the antenna array structure. The feed 14 includes integrated devices 16 such as amplifiers and phase shifters. The common flat ground screen 18 functions to eliminate back radiation of the antenna array and separates a dipole area from a feed area. A low sidelobe level over a relatively wide bandwidth (15-20%) can be achieved by this type of antenna array, as the number of elements is large (see, Low Sidelobe Phased Array Antennas by H. E. Schrank, IEEE APS Newsletter, 25, pp. 5-9). In this way these printed dipole array antennas are widely used in many applications.
However, there are various technical problems of the conventional dipole array that can occur.
A first problem is that a big wind-loaded area can be present from a face direction. This is caused by a solid ground screen. In order to reduce the wind-loaded area, special radomes are typically used, generally increasing the cost of an antenna system.
A second problem is that bandwidth and wide-angle scan limitations can exist due to a mutual coupling phenomena. The mutual coupling is one of the main factors which limit a wideband antenna array operation. In the H-plane the mutual coupling is proportional to 1/.gamma. and in the E-plane to 1/.gamma..sup.2 wherein .gamma. is the distance between dipoles. The mutual coupling in the H-plane is more significant than in the E-plane (see, The Ultimate Decay of Mutual Coupling in a Planar Array Antenna by P. W. Hannan, IEEE Trans., v. AP-14, March 1966, pp. 246-248). In this regard, it is very important to decrease mutual coupling in the H-plane. The mutual coupling can produce an impedance mismatch in a scan area, can reduce a bandwidth and scan angles, and in the case of a relatively small array, can increase sidelobes (see, Phased Array Antenna Handbook, Chapter 6).
A third problem is that the element pattern of a dipole in the array is far from an ideal "top-flat" element pattern with a constant level at a scan angle and a zero level at other angles. In the top-flat element pattern, scan losses are minimized and grating lobes are suppressed. Use of top-flat radiators, for instance, sharp dielectric bars, typically allows a dramatic reduction in the number of elements and the cost of a phased array. Further, the top-flat element pattern is very useful in a fixed-beam antenna array, because of suppression of far sidelobes.
A fourth problem is that quite different parameters can be present in the edge dipoles from those in central dipoles (see Phased Antenna Handbook, p.330). The parameters can include element pattern, impedance, and polarization properties. This edge phenomenon can result in the increase of back lobe and sidelobe, especially in a small array, such as where the number of elements is from 4 to 100.
Lastly, a fifth problem is that in the case of an active array, a ground screen can hinder effective cooling of an active device like a high power amplifier due to poor ventilation.
U.S. Pat. No. 3,587,105 to Neilson entitled Picture Framed Antenna, discloses a folded dipole antenna is provided by means of three circuit boards disposed in three hinged picture frames forming a horizontal array in which the antenna pattern on the circuit boards is made electrically continuous through connections in the hinges of the picture frames.
U.S. Pat. No. 3,681,769 to Perrotti et al. entitled Dual Polarized Printed Circuit Dipole Antenna Array, disclose an antenna array is provided by stacking two PC boards in a superimposed relationship above a housing acting as a ground plane. Each of these two PC boards contain thereon a symmetrical arrangement of photo etched or printed mat-strip power division networks and dipole elements providing linear polarization, the dipole elements on one PC board being oriented with the dipole elements on the other PC board to provide orthogonal linear polarizations. A ground plane for the dipole elements on the upper PC board is provided by parallel, spaced conductive members in a superimposed, parallel relationship with the dipole elements of the upper PC board. In one embodiment, the ground plane conductive members are provided by conductive strips on a third PC board disposed between the first two PC boards. In another embodiment, the same third PC board is disposed between the lower PC board and the housing ground plane therefore. In a third embodiment, the ground plane conducive members are formed as ridges on the housing ground plane.
U.S. Pat. No. 3,681,771 to Lewis et al. entitled Retroflector Dipole Antenna Array And Method of Making, disclose a method of making an antenna array and an antenna array apparatus of a wide angle retroreflector is provided in which a printed circuit board has a plurality of antenna elements etched on one side thereof and a ground plane on the other separated by dielectric material of a predetermined thickness. Baluns are disclosed as being attached through the printed circuit board to each antenna element and to the ground plane and transmission lines of equal length connect spaced pairs of antenna elements utilizing the balun and matching the transmission line to the antenna element.
U.S. Pat. No. 4,360,816 to Corzine entitled Phased Array of Six Log-periodic Dipoles, discloses a direction finding antenna for actuated direction finding over broad conuous frequency spectrums, independently of polarization, including a phased array of six log-periodic dipole antennas with loaded elements.
U.S. Pat. No. 4,471,493 to Schober entitled Wireless Telephone Extension Unit With Self-Contained Dipole Antenna, discloses a remote unit for use in a wireless extension telephone system having a self-contained dipole antenna. Utilizing the construction of the telephone instrument housing one element of the dipole is included in a planar element that functions normally to direct sound to a self-contained microphone and the other element of the antenna is a static shield used to protect components a printed circuit board included within the extension unit.
U.S. Pat. No. 4,590,614 to Erat entitled Dipole Antenna For Portable Radio, discloses a dipole antenna for a portable radio is contained completely within the insulated housing of the transceiver. The dipole antenna is formed as two conductive surfaces electrically isolated from each other but disposed on the same printed circuit board of the transceiver circuit which supports the circuit modules. The two dipole halves are connected to each other by means of a dipole tuning circuit. The conductive tracks of the transceiver circuit are interrupted at a location which divides as few tracks as possible. The interrupted tracks are bridged together by high-impedance resistors.
U.S. Pat. No. 5,313,218 to Busking entitled Antenna Assembly, discloses an antenna assembly that includes a dipole antenna and a monopole antenna having substantially perpendicular polarization directions. The dipole antenna is provided with a balun a portion of which serves as a backplane for a microstrip transmission line which transmits RF signals. The microstrip transmission line includes a first portion connected to a coaxial feed cable, a second portion having its ends respectively connected by a first switch to the monopole antenna and a second switch to the balun portion and third portion when the switches are closed to render the monopole antenna operative, the third portion serves to detune the dipole antenna. The assembly it is disclosed can be formed as a two-sided printed circuit board.
U.S. Pat. No. 5,495,260 to Couture entitled Printed Circuit Dipole Antenna, discloses a paging receiver including a printed circuit board on which receiving circuitry is mounted. The printed circuit board includes a plurality of conductive runners which form a dipole antenna for providing radio frequency signals to the receiving circuitry. First and second elongated runners are disclosed as being plated on a first surface of the printed circuit board along a single axis. Third and fourth elongated runners are plated on a second surface of the printed circuit board parallel to and beneath the first and second elongated runners, respectively. The first and third runners are electrically coupled via a first plated hole from a first monopole element of the dipole antenna for providing the signals to the receiving circuitry, and the second and fourth runners are electrically coupled via a second plated hole to from a second monopole element of the dipole antenna.
U.S. Pat. No. 5,686,928 to Pritchett et al. entitled Phased Array Antenna For Radio Frequency Identification, disclose a multi-element, H plane, phased, dipole array antenna, wherein two printed wiring boards feed and physically support the dipole antenna elements. The phase and spacing of the dipole elements establish the radiation elevation angle, and a planar metallic reflector, spaced on the order of a half wavelength of the RF signal from the dipole array, interacts with the dipole-element pattern, to provide wide angle azimuth gain.
U.S. Pat. No. 5,828,342 to Hayes et al. entitled Multiple Band Printed Monopole Antenna, disclose a printed monopole antenna including a first printed circuit board having a first side and a second side, a first monopole radiating element in the form of a conductive trace formed on a side of the first printed circuit board, and a second monopole radiating element in the form of a conductive trace positioned adjacent the first monopole radiating element, wherein the first monopole radiating element is resonant within a first frequency band and the second monopole radiating element is resonant within a second frequency band. In order for the first and second radiating elements to be resonant within different frequency bands, the conductive traces for each are disclosed to have different electrical lengths. No direct electrical connection is disclosed to exist between the monopole radiating elements, but the second radiating element dominates at a frequency in which the second radiating element is approximately a half-wavelength so that coupling with the first radiating element occurs. The first and second monopole radiating elements are formed on the same side of the first printed circuit board, separate sides of the first printed circuit board, or on separate printed circuit boards.